1. Field of the invention
This invention relates to a capacitor discharge circuit in general and particularly to a discharge circuit constructed by a combination of a mechanical switch and an electronic switch and used for discharging a high voltage-large capacity capacitor.
2. Description of the Prior Art
In recent years, it has been reported that the demand for an extremely small apparatus including a high voltage-large capacity capacitor and capable of being carried by hands is increasing year by year. Among such apparatuses, there are a defibrillator (or resuscitator) for emergency use, a machine tool using a laser, a medical instrument and the other type of apparatus which includes a high voltage-large capacity capacitor and utilizes the instantaneous discharge of a large amount of electric charge from the capacitor.
An oil-sealed type switch has been customarily used as a switch for instantaneously discharging a comparatively large amount of electric charge, but it is not suitable for use as portable type because of such drawbacks as it becomes rether heavy due to the weight of oil and a container thereof, and the oil is liable to drop out of the container due to vibration. On the contrary, an air-open type switch is light in weight, but has disadvantages that arc discharge occurs violently at the time of on-off operation and the wear and tear of contacts is remarkable. A silicon controlled rectifier or thyristor is well known as an electronic switch which is not accompanied with any arc discharge at the time of on-off operation, but some amount of leakage currents may exist even when it is not conducting. Assuming now that the thyristor is used, by itself, as a switch for a capacitor discharge circuit in the defibrillator, for example, it may often lead to very serious accidents. The defibrillator is a device which applies the current at high voltages from the capacitor to the heart of human being in a syncopic state for a short period of time in order to bring him back to consciousness, so that the leakage current through the thyristor, if any, is dangerous since the electric charge from the capacitor may be storaged between both electrodes even when the circuit is not in use.